xen: don't compile pv-specific parts if XEN_PV isn't configured

xenbus_client.c contains some functions specific for pv guests.
Enclose them with #ifdef CONFIG_XEN_PV to avoid compiling them when
they are not needed (e.g. on ARM).

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>